A commonly utilized electrolytic cell for the manufacture of aluminium is of the classic Hall-Heroult design, utilizing carbon anodes and a substantially flat carbon-lined bottom which functions as part of the cathode system. An electrolyte is used in the production of aluminium by electrolytic reduction of alumina, which electrolyte consists primarily of molten cryolite with dissolved alumina, and which may contain other materials such as fluospar, aluminium fluoride, and materials such as fluoride salts. Molten aluminium resulting from the reduction of alumina is most frequently permitted to accumulate in the bottom of the receptacle forming the electrolytic cell, as a metal pad or pool over the carbon-lined bottom, thus forming a liquid metal cathode. Carbon anodes extending into the receptacle, and contracting the molten electrolyte, are adjusted relative to the liquid metal cathode. Current collector bars, such as steel are frequently embedded in the carbon-lined cell bottom, and complete the connection to the cathodic system.
While the design and size of Hall-Heroult electrolytic cells vary, all have a relatively low energy efficiency, ranging from about 35 to 45 percent depending upon cell geometry and mode of operation. Thus, while the theoretical power requirement to produce one kilogram of aluminium is about 6.27 Kilowatt hours (KWh), in practice power usage ranges from 13.2 to 18.7 KWh/Kg, with an industry average of about 16.5 KWh/Kg. A large proportion of this discrepancy from theoretical energy consumption is the result of the voltage drop of the electrolyte between the anode and cathode.
As a result of the above, much study has gone into reduction of the anode-cathode distance (ACD). However, because the molten aluminium pad which serves as the cell cathode can become irregular and variable in thickness due to electromagnetic effects and bath circulation, past practice has required that the ACD be kept at a safe 3.5 to 6 cm to ensure relatively high current efficiencies and to prevent direct shorting between the anode and the metal pad. Such gap distances result in voltage drops from 1.4 to 2.7 volts, which is in addition to the energy required for the electrochemical reaction itself (2.1 volts, based upon enthalpy and free energy calculations). Accordingly, much effort has been directed to developing a more stable aluminium pad, so as to reduce the ACD to less than 3.5 cm, with attendant energy savings.
Refractory hard materials (RHM), such as titanium diboride, have been under study for quite some time for use as cathode surfaces in the form of tiles, but until recently, adherent RHM tiles or surface coatings have not been available. Titanium diboride is known to be conductive, as well as possessing the characteristic of being wetted by molten aluminium, thus permitting formation of very thin aluminium films.
The use of a very thin aluminium film draining down an inclined cathode covered with an RHM surface, to replace the unstable molten aluminium pad of the prior art, has been suggested as a means to reduce the ACD, thus improving efficiency, and reducing voltage drop. However, attempts to achieve such goals in the past have failed due to the inadequacy of available RHM surfaces, and the inability to overcome the difficulty of providing a sufficient supply of dissolved alumina to the narrowed ACD (as small as 1.5 cm). Thus, problems of alumina starvation occur at minimal ACD, including excessive and persistant anode effects. Overfeeding alumina to prevent these problems has resulted in deposits of sludge (mucking), which can clog the cell and restrain its operation.
U.S. Pat. No. 4,602,990 by Boxall et al. discloses a design for a drained cathode cell in which the cathode slope and inter anode distances are arranged so that the balance between buoyancy-generated bubble forces from the inclination and the flow resistance will result in a net motion of the bath to provide the required alumina supply. The rate of flow in the bath circulation loop through the anode-cathode gap (ACD), a bath replenishment zone (a channel where alumina is added to the bath) and return channel between the anodes is primarily controlled by the anode-cathode slope, the ACD gap and the design of the space between adjacent anodes. This patent provides the design specifications to ensure sufficient flow of the bath through the ACD gap to transport an adequate supply of dissolved alumina for the electrolysis reaction within the same ACD gap. In an aluminium reduction cell with sloped anode and cathode faces, the gas formed at the anode face will travel upward along the inclination. In turn, these anode gases will drive the bath in the ACD gap in the same direction. This action generates the forces required to produce the desired bath motion in the electrolysis cell operating at a reduced ACD spacing.
A number of cell designs, such as in the Kaiser-DOE sloping TiB2 cathode tests reported under Contract DE-ACO3-76CS40215, and as used in other published reports and patents including Boxall et al. have not achieved the expected voltage reduction corresponding to the reduction in the ACD gap. This problem is common to a number of different RHM cathode designs incorporating plates, cylinders, vertical and horizontal rods, inverted cups and a packed bed. The RHM cathode slope at a 15 k Amp pilot cell in the Kaiser DOE project was increased from 2 degrees to 5 degrees from horizontal in an attempt to provide more effective gas evolution and electrolyte mixing in the ACD gap. Halving the ACD in the 2 degree cathode slope cell gave a 35% reduction in bath resistance instead of the theoretical 50% reduction. This implies that the effective bath resistivity at the lower ACD was about 30% higher than at the higher ACD. Kaiser ascribed the increased bath resistivity at low ACD's primarily to an increasing void fraction of anode gas as the ACD is decreased. Changing to the 5 degree cathode slope cell did not improve on this detrimental increase in bath resistivity at reduced ACD's.
During the operation of all drained RHM cathode cells, the anode face shape will burn to conform to the shape of the underlying rigid cathode face. This phenomena is referred to as "anode shaping". A similar effect is observed in conventional metal pad aluminium reduction cells where the cell magnetics produce a stable heave in the metal pad surface.
In operation of an aluminium smelting cell, gas bubbles, primarily carbon dioxide, develop on the carbon anode faces as a result of the electrolysis reaction taking place within the cell. These bubbles must find their way out of the ACD gap and then be discharged from the bath electrolyte. In a conventional cell with horizontal anode and cathode faces, the gas bubbles will move in a somewhat random fashion and are eventually discharged along the nearest anode edge. In a drained RHM cathode cell the inclined anode face results in a predominant movement of the gas bubbles upwards along the length of the anode slope. This directed flow of the anode gas bubbles produces the desired bath flow in the ACD. However, the distance between the position of initial formation of the gas bubbles and the exit point from the ACD gap may be quite lengthy and the bubble volume will lend to accumulate with distance under the anode. At reduced ACD's these large gas bubbles increase the bath resistivity and may protrude through the ACD gap to contact or be in close proximity to the aluminium wetted cathode surface. Since drained RHM cathode cells result in a thin film of aluminium wetting the RHM cathode surface, rather than the deeper molten aluminium `pad` characteristic of conventional cells, any disturbance of the film, such as may be caused by undesirable bubble accumulation, will result in a degradation of the performance of the cell as well as redissolution of aluminum into the bath.
Houston et al. (Light Metals pp 641-645, 1988) report a significant increase in the effective bath resistivity for a commercial scale drained cathode cell operating at ACD values down to 1 cm. Since current efficiencies for full scale drained cathode cells have not been reported, it is unknown if this close proximity or contact of the oxidizing anode gases with drained cathode will reduce current efficiency and/or cause damage to the wetted RHM surfaces. Serious loss of current efficiency is observed in conventional aluminium smelting cells when operated at reduced ACD values. Furthermore, bath circulation rates in such drained cathode cells have been found to be somewhat higher than desired, resulting in an undesirable increase in turbulence at the upper end of each anode and creating conditions having the potential to cause further disturbance of the aluminium film and erosion of the cathode coating at these points.
Also included in the patent literature is U.S. Pat. No. 3,501,386 Johnson. The essence of this disclosure is the provision of anodes with shaped lower surfaces in an otherwise standard cell having a planar cathode to expedite the removal of gases and minimize recombination with the metal. Gases are vented towards the shortest escape distance from under the anode. In the process of escaping, the bubble lift action produces an induced electrolyte flow in preferred directions, which assists with bubble removal and electrolyte circulation.
Johnson suggests that the shaping of the anode can be achieved by making the anodes less dense or of greater electrical conductivity at specific locations. Workers familiar with anode manufacture indicate the likelihood of significant practical difficulties in achieving appropriate density variations: mismatching at the boundary between the different regions is likely to produce strength and thermal shock problems, quite apart from the additional processing steps needed to engineer these special anodes. In addition, anode fabrication of this type would be likely to be extremely expensive.
Johnson alternatively suggests that tilting and burning of the anode groupings will provide a means of maintaining a sloped surface underneath the anode. As an initially-sloped anode surface is levelled by burning to the flat cathode profile, so the anode group is tilted in the opposite direction to re-expose a newly-sloped surface. The process is repeated as frequently as every 1-4 hours.
Whilst in principle this approach may seem to be workable, the following factors indicate that it would be largely impractical or unworkable:
Aluminium reduction in cells operating near 1000.degree. C. requires that a frozen crust of electrolyte, together with a loose layer of crushed bath or alumina, be formed on the top of the cell to reduce heat losses in order to maintain a strict heat balance (a critical issue), to restrict the loss of volatile components from the electrolyte, and to provide some oxidation protection for the carbon anodes. The continual tilting of the anode group will produce extensive cracking of the crust layer and lead to large amounts of loose cover falling into the bath. The former effect will degrade the insulating capacity of the frozen crust, requiring the input of more energy to the cell to maintain its heat balance and thus decreasing the overall energy efficiency contrary to a main claim of the inventor. The second effect will produce excessive solid deposits (sludge) on the base of the cell which are notoriously difficult to remove and also require extra energy input. The solid deposits also disrupt the equilibrium electromagnetic fields in the cell, thus disturbing the mobile metal pad and increasing the likelihood for metal fog formation and a consequential lowering of current efficiency, contrary to the claims for improved current efficiency.
Very high electrical currents are used for aluminium electrolysis (ca 150-300,000 Amps at anodic current densities of 0.7-1.0 A/cm.sup.2). The electromagnetic effects caused by the interaction of the electrical and induced magnetic fields generate an equilibrium metal pad profile and degree of metal movement. The equilibrium profile of the metal surface is set by the interaction of the whole electromagnetic force field. Cells are specifically designed with great care to achieve a balance in the forces so that metal circulation and wave formation are kept to an acceptable level.
The continual tilting of the anode group will cause repeated changes to the electromagnetic force field with a consequent destabilization of the equilibrium metal pad profile, leading to an increase in the motion of the metal surface. Furthermore the tilting action will act to concentrate the applied current along one edge of the anode, thus dramatically increasing the local current intensity which in turn, leads to a localized influence on that bit of metal closest to the anode edge, producing a changing and asymmetric force on the metal, destroying the equilibrium metal profile. These combined influences increase the overall likelihood of metal fog formation and back reaction, contrary to the claims. The very changeability of the force fields produces an environment of uncertainty regarding the behaviour of the metal pad, which no operator would choose to accept.
The tilting motion brings one edge of the anode much closer to the metal pad surface for a time. The normal practice in conventional aluminium reduction cells is to maintain a good distance between the anodes and the mobile metal pool to avoid contact with the waves that often exist at the metal surface. During tilting, the change for contact is increased with a resultant unstable cell voltage and intermittent short circuiting, leading to poor current and energy efficiency. To avoid this situation, the anode cathode distance would need to be increased which in turn would increase the cell voltage and diminish the stated voltage benefit.
It would seem, from the absence of working examples in the Johnson patent, doubtful that even a pilot scale cell has been operated according to this invention. In the 20 years since the patent has been published, there has been no record of its commercial use, which may be regarded as a good indication of its fundamental unworkability.
In the U.S. Pat. No. 4,405,433 to Payne, there is provided a very steeply sloping anode and cathode structures, with slopes of around 60.degree. to 85.degree. (i.e. nearly vertical). The aim of the invention is to provide for enhanced bubble removal from the ACD and to thereby achieve a decrease in the bubble voltage component. A second aim is to provide a means for the ready replacement of the fragile and easily damaged RHM materials.
The disadvantages of this patent are as follows:
Payne specifically states (column 4, lines 27-43) that bubble problems occur in drained cathode cells employing low slops and that steep slopes are needed to enhance the bubble release. The results achieved by the present invention, as detailed below, show that improved cell voltages can be achieved even with shallow slopes at low ACD's.
It is necessary to run the Payne cell with a liquid bath surface (i.e. crust free) to enable the pivoting anodes to move. This is undesirable because of the splashing of the molten bath and the loss of the volatile electrolyte. Because of the splashing--which is actually intensified due to the gas pumping effect of this electrode orientation--it will be almost impossible to prevent some crust from formings. The crust so formed will then interfere with the anode movement. Furthermore it would be expected that the superstructure construction materials (usually steel) will be subjected to much more severe corrosion conditions due to the open nature of the cell: the bath and its vapour are both extremely corrosive and combined with the hotter ambient temperatures in the absence of a protective crust will exacerbate the situation.
The patent does not take into account that the nearly vertical orientation of the electrodes concentrates all the bubble induced turbulence at the top end of the electrodes, thus producing a highly turbulent regime still within the ACD, which would be conducive to a number of detrimental effects as noted herein in our application. The present invention specifically seeks to reduce these effects.
Both of the last two patents require quite radical departures from and changes to conventional reduction cell superstructures, thus requiring costly rebuilding of cells, adjustments to in-plant routine, and/or alterations to the processing and installation of anodes. The present invention has the advantages of being able to use the existing anode processing stream and only minor changes to the cathode shape which are easily implemented during the normal cathode construction phase.
It is against this background that the present invention has developed.